The present invention concerns an enhanced hydrocarbon recovery process using foams mainly containing at least one surfactant with perfluoric group to improve gas sweeping in enhanced oil recovery.
The prior art is illustrated by the European patent 173176 which discloses the use of perfluoric compounds to control the mobility of water in porous or fractured medium. They are water thickeners acting as viscosity increasing agent for an injected single aqueous phase, but not as foaming agent.
Compounds with perfluoric groups are also known, particularly from U.S. Pat. No. 4,108,782, as giving foams for use alone or as mixture in processes for treating oil wells. They provide, in particular, for an efficient cleaning of sludges and deposits recovered after acid well treatments. Moreover, the compound disclosed in this U.S. patent is a rather poor foaming agent.
The invention is particularly applicable to enhanced oil recovery from a subsurface reservoir wherein, as a result of the low density of the sweeping gas, of its greater mobility and/or of the permeability difference of the strata to be swept, the gas tends to follow preferential paths, particularly at the upper part of the reservoir.
As a matter of fact, in enhanced oil recovery operations, during the gas injection, the oil is satisfactorily moving in zones effectively swept by the gas. Unfortunately, due to its specific gravity, the latter tends to flow preferentially towards the top of the reservoir, particularly when the oil contained therein is viscous. After the injected fluid has reached the one or more producing well(s), the sweeping becomes ineffective in zones where oil is still present.
The same is true for heterogeneous or fractured fields wherein more permeable drains are responsible for the premature gas release, while the oil tends to remain into the less permeable layers.
In order to improve the sweeping rate, it may be considered to block the gas access to zones preferentially flushed by the sweeping fluid, either by gravity or by permeability difference effect.
It is known, particularly from U.S. Pat. No. 4,086,964, to use foams, produced from such foaming surfactants as sulfonates with hydrocarbon chains, which generate with the injected gas, a foam which may travel within the porous medium and block the gas preferential paths.
However, when contacted with a hydrocarbon phase of the field, the hydrocarbon surfactants present in the foams tend to migrate to the oil phase. Their concentration decrease in the film-forming foam results in a mechanical degradation of the latter. The "foam" structure cannot be maintained; it disappears and is replaced by a water+surfactant/oil/gas mixture. Then, these three phases tend to take separate paths within the porous medium. Accordingly, the method becomes quite ineffective.
In order to cope with this disadvantage, searches have been conducted for obtaining products keeping their foaming properties within the field and forming mechanically stable foams in the presence of a hydrocarbon phase.